Microporous membranes can be used as battery separators in, e.g., primary and secondary lithium batteries, lithium polymer batteries, nickel-hydrogen batteries, nickel-cadmium batteries, nickel-zinc batteries, silver-zinc secondary batteries, etc. When microporous membranes are used as battery separators, particularly lithium ion battery separators, the membranes' performance significantly affects the properties, productivity and safety of the batteries. Accordingly, the microporous membrane should have suitable mechanical properties, heat resistance, permeability, dimensional stability, shutdown properties, meltdown properties, etc. It is desirable for the batteries to have a relatively low shutdown temperature and relatively high temperature stability for improved battery-safety properties, particularly for batteries that are exposed to high temperatures during manufacturing, charging, re-charging, overcharging, use, and/or storage. Improving separator permeability generally leads to an improvement in the battery's power and capacity. Low shutdown temperature is desired for improved battery safety, particularly when the battery is operated under overcharge conditions. Improved separator storage stability is desired because deterioration of separator at high temperature causes decreased battery voltage.
In general, multilayer microporous membrane separators produced from polypropylene can have an increased meltdown temperature. Such separators may include polyethylene in order to provide a relatively low shutdown temperature, particularly when the polyethylene has a significant amount of terminal unsaturation. See, for example, WO97-23554A and JP2002-338730A. Typically, shutdown temperatures are determined by increasing the temperature of the separator at a relatively slow rate to accurately observe the shutdown transition. But in a battery, heating may occur rapidly, for example during an overcharge. Under such conditions the shutdown response of such membranes under rapid heating conditions may not be satisfactory and the cell may experience relatively rapid heating to temperatures exceeding the separator shutdown temperature. If the separator cannot shutdown before the rupture temperature is reached, the battery can fail.
Thus, a separator that has a shutdown temperature lower than conventionally attainable while maintaining relatively high rupture characteristics would provide an increased margin of safety during such rapid heating conditions. Likewise, separators that have improved shutdown response under rapid heating conditions would be useful.